1) Field of the Invention
This invention relates to a flexible cable termination for terminating a submarine cable to a submerged repeater housing or the like.
2) Discussion of the Prior Art
Underwater long distance modern-day cable systems such as TAT-8 and P-TAT systems utilise a bend limiting device where the cable joins the submerged repeater housing.
Laying and recovery of such a cable imparts high tensile forces in the cable. When a repeater passes around a sheath or winding drum, non equi-axial loading across the cable strength wires may occur due to the severe local bending imposed on the cable at its termination with the repeater. This would inevitably lead to failure of cable components at loads well below the tensile strength of the cable itself.
The purpose of the bend limiter is to equalise these forces by providing a semi-flexible element at each end of the repeater. This device provides a working solution to the problem of bend control at the termination of the cable and the repeater. British patent 2142788 describes this device known as the Armadillo bend limiter. It comprises a segmented structure in the form of a tube made from a number of annular plates, each plate connected to its neighbour with bolts. The structure is tapered externally by providing segments in a range of sizes. A resilient sleeve is provided within the bend limiter to exclude materials which might jam or abraid the bend limiter parts.
In the Armadillo bend limiter, the bolts are of sufficient length to provide a predetermined gap between the adjacent plates. When a straight pull is applied the bend limiter extends to the limit of the bolt restraints. When it is deflected the plates make contact on the inside of the curve whilst they are restrained on the outside by the bolts providing a limited bend. The result of this is that there is a certain amount of play or slackness in both the axial and rotational direction, and this slackness has to be accommodated in the design of the tail tube which links the cable termination to the repeater.
In an ideal situation laying and recovery techniques employed with shipboard installations of for example TAT-8 and P-TAT, require the cable and repeater to pass around the hauling capstan and over the bow and stern sheaths. Whether this is feasible depends to a large extent on the rigid length of the repeater and the physical clearance around the hauling capstan. Similar space constraints are not as severe around the bow and stern sheaths.
An alternative arrangement is known as the Gimbal arrangement and an example is shown in European Patent Application 0268210 and British Patent Application 2197539. This arrangement is satisfactory for short repeater housings e.g. up to 1.5 meters because the angle of deflection is limited to about 58.degree.. For longer housings e.g. about 3 meters the deflection is insufficient.
An alternative solution to the bend-limiter and the gimbal arrangement, although not the first, was proposed about 40 years ago and is shown in British Patent 634483. In this proposal the problem of excessive bend on the cable where it joins the repeater housing is avoided by making the whole repeater housing itself flexible. Here the repeater housing is built up of a plurality of comparatively short tubes, each accommodating a complete unit of the repeater, the tubes being connected to one another by ball and socket joints and which have passages through the joint components to prevent interconnection of the individual repeater units.
This particular form of flexible repeater housing has never we believe been used commercially, owing probably to the expense of manufacture and the difficulty of providing a watertight, gas tight and pressure resistant housing for the amplifier components of the repeater. This aspect is touched upon in line 35, page 3, where it is suggested that the repeater apparatus is itself enclosed in a single continuous sheath of copper. It is not clear however if this is pressure resistant or if the housing is intended to be so, in which case great reliance would need to be placed on the seals alluded to in line 24 on page 3.
Flexible repeater housings were however used commercially many years ago for analogue systems where they were built into the cable structure by wire reinforcing elements laid over the repeater element.
Here the repeater elements are pressure-resistant, and are surrounded by polythene, which provides the water resistance in turn surrounded by armour wires providing the required tensile strength across the repeater. Such an arrangement has not been used for many years and certainly not extended to optical submarine systems.
It is an object of the present invention to provide a flexible cable termination providing flexure over a large angle in excess of 58.degree. in a simple and cost effective manner, and enabling the problem of slack associated with previous bend-limiter proposals to be minimised if not eliminated.